1. Field of the Invention
The present invention relates to a prism sheet and more particularly to a prism sheet structure for enhancing light luminance and a liquid crystal display device that includes the prism sheet.
2. Description of the Related Art
Prism sheet is used in display devices. For instance, a prism sheet may be used with the backlight source for an LCD device, a luminance-enhancing film, a light-reflecting film, etc. A “prism sheet” commonly refers to a sheet including one surface that is made of a polyester film having adhesive property as the base film and an ultraviolet-hardened resin film laminated on the base film. The prism sheet is provided mainly to enhance the luminance of a light-emitting device (e.g., a liquid crystal display device) by narrowing the viewing angle of a light that has passed through the diffusion plate of the light-emitting device.
Various attempts have been made to enhance the luminance of liquid crystal displays (“LCD”) device. Among these attempts, elevating the supply voltage has been proved to be an effective way to achieve the intended luminance enhancement. However, elevating the supply voltage requires an increased battery capacity which undesirably adds to the weight of the device and decreases battery life.
To this end, various methods for enhancing the luminance while lowering the supply voltage have been studied.
One of these methods, which is disclosed in U.S. Pat. No. 4,906,070, includes a thin flexible film made of a polymeric material including a structured surface and an opposite smooth surface. In the flexible film, the structured surface includes a linear array of miniature substantially right angled isosceles prisms arranged side-by-side to form a plurality of peaks and grooves. In addition, the perpendicular sides of the prisms make an angle of approximately 45°. However, this flexible film was proposed for internally reflecting light, and was not intended for application to an LCD device.
FIG. 1 is a perspective view of a right angled isosceles prism sheet 10 applied to a conventional LCD device. Referring to FIG. 1, right angled isosceles prisms 12 are arranged in parallel with each other in one direction (ex. X-direction) of a surface from which light is emitted. A surface placed adjacent to the surface on which the prisms are formed is a smooth plane 14. Each of the prisms include an incline plane C1 (see FIG. 2A below) of these prisms 12 that has an angle of approximately 45° with respect to a normal line (Y) perpendicular to this smooth plane 14.
As shown in FIG. 2A, if a light strikes a right incline plane C2 of the prism sheet from within an approximately 2.8° cone, wherein the 2.8° is measured with respect to the normal line (Y), this incident light is totally internally reflected as shown by paths of 1 and 2. This totally internally reflected light never escapes the prism and is lost.
Similarly, if a light strikes a left incline plane C1 of the prism sheet at an angle within an approximately 2.8° cone with respect to the normal line (Y), this incident light is totally internally reflected along paths 3 and 4. This light never escapes the prism and is lost.
The above two facts mean that if a light strikes a surface of a prism sheet at an angle that is less than 2.8°, light extraction efficiency is lowered. Consequently, the front luminance is lowered.
FIG. 3 is a view showing a measurement result of luminance distribution of light passing through the diffusion plate in a conventional LCD device, and FIG. 4 is a view showing a measurement result of luminance distribution of light passing through the prism sheet in the conventional LCD device.
Referring to FIG. 3, when a distribution of the luminance is measured with varying the viewing angle at a point on the diffusion sheet, a light amount of approximately 55.5% with respect to a total light amount is emitted at a viewing angle of approximately ±34° (“A”). This, as shown in FIG. 4, means that approximately 55.5% of the light that is incident at an incident angle of an approximately ±34° on the prism sheet is refracted or emitted toward the front side where an image is displayed.
Since light having an angle of incidence less than approximately 2.8° with respect to the normal line is not emitted, the amount of such light should be minimized to prevent the luminance from being lowered.
In order to prevent the luminance from being lowered, there were the following prior arts.
Japanese Patent Laid-Open Publication No. 8-320405 discloses a prism sheet and a back light without impairing front luminance as a back light and causing a bright and dark pattern by composing each prism surface of a wavy surface having a specified height difference for every prism unit. In this prism sheet, the prism surface of each prism unit is formed with a surface of waviness having a height difference (D) higher than 1 μm.
Japanese Laid-Open Publication No. 7-230002 discloses a directional back light which has its directivity improved. The transparent sheet is formed by arraying many shape units 8, each formed by connecting two convex curved columnar surfaces having generating lines parallel to each other in right-left symmetrical relation at right angles to the generating lines, on one surface, and the respective tangential surfaces and planes cross each other at a 180° angle.
Japanese Laid-Open Publication No. 6-250182 discloses a prism sheet that is used for enhancing the luminance and does not generate stripe patterns. In this prism sheet, many projecting parts are formed on the front surface of the prism sheet to be used by arranging the sheet on the exit surface side of the surface light source device and forming the entire part or a part of these projecting parts as an embossed surface.
Japanese Patent Laid Open Publication No. 8-254606 discloses a lens sheet enabling to eliminate unnecessary light of an incline direction without lowering the luminance in the normal direction. The lens sheet has a lens array layer in which a plurality of unit lens parts are arranged in one-dimension or two-dimension at the light exit side of a transparent base part and are formed in the form of a projecting shape on the light exit side. The section (main cut surface) of the one direction or the two directions orthogonal with each other is formed in a pentagonal shape. In such a case, the angle θ4 formed by the two sides on the base part side of the unit lens parts is steeper (θ3>θ4) than the angle (i.e., vertical angle) θ3 formed by the two sides on the vertex side.
While the aforementioned prior arts achieve the objects of preventing the occurrence of the stripe pattern and enhancing the luminance, they do not solve a problem related to utilization of the light that is incident with an angle below 2.8° with respect to the normal line perpendicular to the emitting plane. As described later, approximately 70% of the light that is incident on the prism sheet is incident with an angle below 50°. To this end, it is desirable to enhance the utilization efficiency of the light that is incident with the aforementioned angle below approximately 2.8° so as to increase the luminance.